Observation of Lambda(+)(c) -> nK(S)(0)pi(+)

This is the accepted manuscript made available via CHORUS. The article has been

published as:

Observation of Λ_{c}^{+}→nK_{S}^{0}π^{+}

M. Ablikim et al. (BESIII Collaboration)

Phys. Rev. Lett. 118, 112001 — Published 14 March 2017

DOI:

10.1103/PhysRevLett.118.112001

M. Ablikim1_{, M. N. Achasov}9,e_{, S. Ahmed}14_{, X. C. Ai}1_{, O. Albayrak}5_{, M. Albrecht}4_{, D. J. Ambrose}44_{, A. Amoroso}49A,49C_, 2

F. F. An1, Q. An46,a, J. Z. Bai1, O. Bakina23, R. Baldini Ferroli20A, Y. Ban31, D. W. Bennett19, J. V. Bennett5, N. Berger22,

M. Bertani20A_{, D. Bettoni}21A_{, J. M. Bian}43_{, F. Bianchi}49A,49C_{, E. Boger}23,c_{, I. Boyko}23_{, R. A. Briere}5_{, H. Cai}51_{, X. Cai}1,a_,

4

O. Cakir40A_{, A. Calcaterra}20A_{, G. F. Cao}1_{, S. A. Cetin}40B_{, J. F. Chang}1,a_{, G. Chelkov}23,c,d_{, G. Chen}1_{, H. S. Chen}1_,

J. C. Chen1_{, M. L. Chen}1,a_{, S. Chen}41_{, S. J. Chen}29_{, X. Chen}1,a_{, X. R. Chen}26_{, Y. B. Chen}1,a_{, X. K. Chu}31_{, G. Cibinetto}21A_,

6

H. L. Dai1,a_{, J. P. Dai}34_{, A. Dbeyssi}14_{, D. Dedovich}23_{, Z. Y. Deng}1_{, A. Denig}22_{, I. Denysenko}23_{, M. Destefanis}49A,49C_,

F. De Mori49A,49C_{, Y. Ding}27_{, C. Dong}30_{, J. Dong}1,a_{, L. Y. Dong}1_{, M. Y. Dong}1,a_{, Z. L. Dou}29_{, S. X. Du}53_{, P. F. Duan}1_,

8

J. Z. Fan39_{, J. Fang}1,a_{, S. S. Fang}1_{, X. Fang}46,a_{, Y. Fang}1_{, R. Farinelli}21A,21B_{, L. Fava}49B,49C_{, F. Feldbauer}22_{, G. Felici}20A_,

C. Q. Feng46,a_{, E. Fioravanti}21A_{, M. Fritsch}14,22_{, C. D. Fu}1_{, Q. Gao}1_{, X. L. Gao}46,a_{, Y. Gao}39_{, Z. Gao}46,a_{, I. Garzia}21A_,

10

K. Goetzen10_{, L. Gong}30_{, W. X. Gong}1,a_{, W. Gradl}22_{, M. Greco}49A,49C_{, M. H. Gu}1,a_{, Y. T. Gu}12_{, Y. H. Guan}1_{, A. Q. Guo}1_,

L. B. Guo28_{, R. P. Guo}1_{, Y. Guo}1_{, Y. P. Guo}22_{, Z. Haddadi}25_{, A. Hafner}22_{, S. Han}51_{, X. Q. Hao}15_{, F. A. Harris}42_{, K. L. He}1_,

12

F. H. Heinsius4_{, T. Held}4_{, Y. K. Heng}1,a_{, T. Holtmann}4_{, Z. L. Hou}1_{, C. Hu}28_{, H. M. Hu}1_{, J. F. Hu}49A,49C_{, T. Hu}1,a_{, Y. Hu}1_,

G. S. Huang46,a_{, J. S. Huang}15_{, X. T. Huang}33_{, X. Z. Huang}29_{, Z. L. Huang}27_{, T. Hussain}48_{, W. Ikegami Andersson}50_{, Q. Ji}1_,

14

Q. P. Ji15_{, X. B. Ji}1_{, X. L. Ji}1,a_{, L. W. Jiang}51_{, X. S. Jiang}1,a_{, X. Y. Jiang}30_{, J. B. Jiao}33_{, Z. Jiao}17_{, D. P. Jin}1,a_{, S. Jin}1_,

T. Johansson50_{, A. Julin}43_{, N. Kalantar-Nayestanaki}25_{, X. L. Kang}1_{, X. S. Kang}30_{, M. Kavatsyuk}25_{, B. C. Ke}5_,

16

P. Kiese22_{, R. Kliemt}10_{, B. Kloss}22_{, O. B. Kolcu}40B,h_{, B. Kopf}4_{, M. Kornicer}42_{, A. Kupsc}50_{, W. K¨uhn}24_{, J. S. Lange}24_,

M. Lara19_{, P. Larin}14_{, L. Lavezzi}49C,1_{, H. Leithoff}22_{, C. Leng}49C_{, C. Li}50_{, Cheng Li}46,a_{, D. M. Li}53_{, F. Li}1,a_{, F. Y. Li}31_,

18

G. Li1_{, H. B. Li}1_{, H. J. Li}1_{, J. C. Li}1_{, Jin Li}32_{, K. Li}13_{, K. Li}33_{, Lei Li}3_{, P. R. Li}7,41_{, Q. Y. Li}33_{, T. Li}33_{, W. D. Li}1_,

W. G. Li1_{, X. L. Li}33_{, X. N. Li}1,a_{, X. Q. Li}30_{, Y. B. Li}2_{, Z. B. Li}38_{, H. Liang}46,a_{, Y. F. Liang}36_{, Y. T. Liang}24_,

20

G. R. Liao11_{, D. X. Lin}14_{, B. Liu}34_{, B. J. Liu}1_{, C. X. Liu}1_{, D. Liu}46,a_{, F. H. Liu}35_{, Fang Liu}1_{, Feng Liu}6_{, H. B. Liu}12_,

H. H. Liu1_{, H. H. Liu}16_{, H. M. Liu}1_{, J. Liu}1_{, J. B. Liu}46,a_{, J. P. Liu}51_{, J. Y. Liu}1_{, K. Liu}39_{, K. Y. Liu}27_{, L. D. Liu}31_,

22

P. L. Liu1,a_{, Q. Liu}41_{, Q. J. Liu}3_{, S. B. Liu}46,a_{, X. Liu}26_{, Y. B. Liu}30_{, Y. Y. Liu}30_{, Z. A. Liu}1,a_{, Z. Q. Liu}22_{, H. Loehner}25_,

X. C. Lou1,a,g_{, H. J. Lu}17_{, J. G. Lu}1,a_{, Y. Lu}1_{, Y. P. Lu}1,a_{, C. L. Luo}28_{, M. X. Luo}52_{, T. Luo}42_{, X. L. Luo}1,a_{, X. R. Lyu}41_,

24

F. C. Ma27_{, H. L. Ma}1_{, L. L. Ma}33_{, M. M. Ma}1_{, Q. M. Ma}1_{, T. Ma}1_{, X. N. Ma}30_{, X. Y. Ma}1,a_{, Y. M. Ma}33_{, F. E. Maas}14_,

M. Maggiora49A,49C_{, Q. A. Malik}48_{, Y. J. Mao}31_{, Z. P. Mao}1_{, S. Marcello}49A,49C_{, J. G. Messchendorp}25_{, G. Mezzadri}21B_,

26

J. Min1,a_{, T. J. Min}1_{, R. E. Mitchell}19_{, X. H. Mo}1,a_{, Y. J. Mo}6_{, C. Morales Morales}14_{, N. Yu. Muchnoi}9,e_{, H. Muramatsu}43_,

P. Musiol4_{, Y. Nefedov}23_{, F. Nerling}10_{, I. B. Nikolaev}9,e_{, Z. Ning}1,a_{, S. Nisar}8_{, S. L. Niu}1,a_{, X. Y. Niu}1_{, S. L. Olsen}32_,

28

Q. Ouyang1,a_{, S. Pacetti}20B_{, Y. Pan}46,a_{, P. Patteri}20A_{, M. Pelizaeus}4_{, H. P. Peng}46,a_{, K. Peters}10,i_{, J. Pettersson}50_,

J. L. Ping28_{, R. G. Ping}1_{, R. Poling}43_{, V. Prasad}1_{, H. R. Qi}2_{, M. Qi}29_{, S. Qian}1,a_{, C. F. Qiao}41_{, L. Q. Qin}33_{, N. Qin}51_,

30

X. S. Qin1_{, Z. H. Qin}1,a_{, J. F. Qiu}1_{, K. H. Rashid}48_{, C. F. Redmer}22_{, M. Ripka}22_{, G. Rong}1_{, Ch. Rosner}14_{, X. D. Ruan}12_,

A. Sarantsev23,f, M. Savri´e21B, C. Schnier4, K. Schoenning50, W. Shan31, M. Shao46,a, C. P. Shen2, P. X. Shen30,

32

X. Y. Shen1_{, H. Y. Sheng}1_{, W. M. Song}1_{, X. Y. Song}1_{, S. Sosio}49A,49C_{, S. Spataro}49A,49C_{, G. X. Sun}1_{, J. F. Sun}15_,

S. S. Sun1_{, X. H. Sun}1_{, Y. J. Sun}46,a_{, Y. Z. Sun}1_{, Z. J. Sun}1,a_{, Z. T. Sun}19_{, C. J. Tang}36_{, X. Tang}1_{, I. Tapan}40C_,

34

E. H. Thorndike44_{, M. Tiemens}25_{, I. Uman}40D_{, G. S. Varner}42_{, B. Wang}30_{, B. L. Wang}41_{, D. Wang}31_{, D. Y. Wang}31_,

K. Wang1,a_{, L. L. Wang}1_{, L. S. Wang}1_{, M. Wang}33_{, P. Wang}1_{, P. L. Wang}1_{, W. Wang}1,a_{, W. P. Wang}46,a_{, X. F. Wang}39_,

36

Y. Wang37_{, Y. D. Wang}14_{, Y. F. Wang}1,a_{, Y. Q. Wang}22_{, Z. Wang}1,a_{, Z. G. Wang}1,a_{, Z. H. Wang}46,a_{, Z. Y. Wang}1_,

T. Weber22_{, D. H. Wei}11_{, P. Weidenkaff}22_{, S. P. Wen}1_{, U. Wiedner}4_{, M. Wolke}50_{, L. H. Wu}1_{, L. J. Wu}1_{, Z. Wu}1,a_,

38

L. Xia46,a_{, L. G. Xia}39_{, Y. Xia}18_{, D. Xiao}1_{, H. Xiao}47_{, Z. J. Xiao}28_{, Y. G. Xie}1,a_{, Yuehong Xie}6_{, Q. L. Xiu}1,a_{, G. F. Xu}1_,

J. J. Xu1_{, L. Xu}1_{, Q. J. Xu}13_{, Q. N. Xu}41_{, X. P. Xu}37_{, L. Yan}49A,49C_{, W. B. Yan}46,a_{, W. C. Yan}46,a_{, Y. H. Yan}18_,

40

H. J. Yang34,j_{, H. X. Yang}1_{, L. Yang}51_{, Y. X. Yang}11_{, M. Ye}1,a_{, M. H. Ye}7_{, J. H. Yin}1_{, Z. Y. You}38_{, B. X. Yu}1,a_,

C. X. Yu30, J. S. Yu26, C. Z. Yuan1, Y. Yuan1, A. Yuncu40B,b, A. A. Zafar48, Y. Zeng18, Z. Zeng46,a, B. X. Zhang1,

42

B. Y. Zhang1,a_{, C. C. Zhang}1_{, D. H. Zhang}1_{, H. H. Zhang}38_{, H. Y. Zhang}1,a_{, J. Zhang}1_{, J. J. Zhang}1_{, J. L. Zhang}1_,

J. Q. Zhang1_{, J. W. Zhang}1,a_{, J. Y. Zhang}1_{, J. Z. Zhang}1_{, K. Zhang}1_{, L. Zhang}1_{, S. Q. Zhang}30_{, X. Y. Zhang}33_{, Y. Zhang}1_,

44

Y. H. Zhang1,a, Y. N. Zhang41, Y. T. Zhang46,a, Yu Zhang41, Z. H. Zhang6, Z. P. Zhang46, Z. Y. Zhang51, G. Zhao1,

J. W. Zhao1,a_{, J. Y. Zhao}1_{, J. Z. Zhao}1,a_{, Lei Zhao}46,a_{, Ling Zhao}1_{, M. G. Zhao}30_{, Q. Zhao}1_{, Q. W. Zhao}1_{, S. J. Zhao}53_,

46

T. C. Zhao1_{, Y. B. Zhao}1,a_{, Z. G. Zhao}46,a_{, A. Zhemchugov}23,c_{, B. Zheng}47_{, J. P. Zheng}1,a_{, W. J. Zheng}33_{, Y. H. Zheng}41_,

B. Zhong28_{, L. Zhou}1,a_{, X. Zhou}51_{, X. K. Zhou}46,a_{, X. R. Zhou}46,a_{, X. Y. Zhou}1_{, K. Zhu}1_{, K. J. Zhu}1,a_{, S. Zhu}1_,

48

S. H. Zhu45_{, X. L. Zhu}39_{, Y. C. Zhu}46,a_{, Y. S. Zhu}1_{, Z. A. Zhu}1_{, J. Zhuang}1,a_{, L. Zotti}49A,49C_{, B. S. Zou}1_{, J. H. Zou}1

(BESIII Collaboration) 50

1 _{Institute of High Energy Physics, Beijing 100049, People’s Republic of China}

2 _{Beihang University, Beijing 100191, People’s Republic of China}

52

3 _{Beijing Institute of Petrochemical Technology, Beijing 102617, People’s Republic of China}

4 _{Bochum Ruhr-University, D-44780 Bochum, Germany}

54

5 _{Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA}

6 _{Central China Normal University, Wuhan 430079, People’s Republic of China}

56

7 _{China Center of Advanced Science and Technology, Beijing 100190, People’s Republic of China}

8

COMSATS Institute of Information Technology, Lahore, Defence Road, Off Raiwind Road, 54000 Lahore, Pakistan 58

9 _{G.I. Budker Institute of Nuclear Physics SB RAS (BINP), Novosibirsk 630090, Russia}

10 _{GSI Helmholtzcentre for Heavy Ion Research GmbH, D-64291 Darmstadt, Germany}

60

2

12 _{Guangxi University, Nanning 530004, People’s Republic of China}

62

13 _{Hangzhou Normal University, Hangzhou 310036, People’s Republic of China}

14 _{Helmholtz Institute Mainz, Johann-Joachim-Becher-Weg 45, D-55099 Mainz, Germany}

64

15 _{Henan Normal University, Xinxiang 453007, People’s Republic of China}

16

Henan University of Science and Technology, Luoyang 471003, People’s Republic of China 66

17 _{Huangshan College, Huangshan 245000, People’s Republic of China}

18 _{Hunan University, Changsha 410082, People’s Republic of China}

68

19 _{Indiana University, Bloomington, Indiana 47405, USA}

20 _{(A)INFN Laboratori Nazionali di Frascati, I-00044, Frascati,}

70

Italy; (B)INFN and University of Perugia, I-06100, Perugia, Italy

21 _{(A)INFN Sezione di Ferrara, I-44122, Ferrara, Italy; (B)University of Ferrara, I-44122, Ferrara, Italy}

72

22 _{Johannes Gutenberg University of Mainz, Johann-Joachim-Becher-Weg 45, D-55099 Mainz, Germany}

23 _{Joint Institute for Nuclear Research, 141980 Dubna, Moscow region, Russia}

74

24 _{Justus-Liebig-Universitaet Giessen, II. Physikalisches Institut, Heinrich-Buff-Ring 16, D-35392 Giessen, Germany}

25 _{KVI-CART, University of Groningen, NL-9747 AA Groningen, The Netherlands}

76

26 _{Lanzhou University, Lanzhou 730000, People’s Republic of China}

27 _{Liaoning University, Shenyang 110036, People’s Republic of China}

78

28 _{Nanjing Normal University, Nanjing 210023, People’s Republic of China}

29 _{Nanjing University, Nanjing 210093, People’s Republic of China}

80

30 _{Nankai University, Tianjin 300071, People’s Republic of China}

31 _{Peking University, Beijing 100871, People’s Republic of China}

82

32 _{Seoul National University, Seoul, 151-747 Korea}

33 _{Shandong University, Jinan 250100, People’s Republic of China}

84

34 _{Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China}

35 _{Shanxi University, Taiyuan 030006, People’s Republic of China}

86

36 _{Sichuan University, Chengdu 610064, People’s Republic of China}

37 _{Soochow University, Suzhou 215006, People’s Republic of China}

88

38 _{Sun Yat-Sen University, Guangzhou 510275, People’s Republic of China}

39 _{Tsinghua University, Beijing 100084, People’s Republic of China}

90

40_{(A)Ankara University, 06100 Tandogan, Ankara, Turkey; (B)Istanbul Bilgi University, 34060 Eyup, Istanbul, Turkey;}

(C)Uludag University, 16059 Bursa, Turkey; (D)Near East University, Nicosia, North Cyprus, Mersin 10, Turkey 92

41 _{University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China}

42 _{University of Hawaii, Honolulu, Hawaii 96822, USA}

94

43

University of Minnesota, Minneapolis, Minnesota 55455, USA

44 _{University of Rochester, Rochester, New York 14627, USA}

96

45 _{University of Science and Technology Liaoning, Anshan 114051, People’s Republic of China}

46 _{University of Science and Technology of China, Hefei 230026, People’s Republic of China}

98

47 _{University of South China, Hengyang 421001, People’s Republic of China}

48 _{University of the Punjab, Lahore-54590, Pakistan}

100

49 _{(A)University of Turin, I-10125, Turin, Italy; (B)University of Eastern}

Piedmont, I-15121, Alessandria, Italy; (C)INFN, I-10125, Turin, Italy 102

50 _{Uppsala University, Box 516, SE-75120 Uppsala, Sweden}

51 _{Wuhan University, Wuhan 430072, People’s Republic of China}

104

52

Zhejiang University, Hangzhou 310027, People’s Republic of China

53 _{Zhengzhou University, Zhengzhou 450001, People’s Republic of China}

106

a_{Also at State Key Laboratory of Particle Detection and Electronics, Beijing 100049, Hefei 230026, People’s Republic of China}

b _{Also at Bogazici University, 34342 Istanbul, Turkey}

108

c _{Also at the Moscow Institute of Physics and Technology, Moscow 141700, Russia}

d _{Also at the Functional Electronics Laboratory, Tomsk State University, Tomsk, 634050, Russia}

110

e _{Also at the Novosibirsk State University, Novosibirsk, 630090, Russia}

f _{Also at the NRC “Kurchatov Institute”, PNPI, 188300, Gatchina, Russia}

112

g _{Also at University of Texas at Dallas, Richardson, Texas 75083, USA}

h _{Also at Istanbul Arel University, 34295 Istanbul, Turkey}

114

i _{Also at Goethe University Frankfurt, 60323 Frankfurt am Main, Germany}

j _{Also at Institute of Nuclear and Particle Physics, Shanghai Key Laboratory for}

116

Particle Physics and Cosmology, Shanghai 200240, People’s Republic of China

We report the first direct measurement of decays of the Λ+

c baryon involving the neutron. The

118

analysis is performed using 567 pb−1 _{of e}+

e− collision data collected at √s = 4.599 GeV with

the BESIII detector at the BEPCII collider. We observe the decay Λ+

c → nKS0π+ and measure

120

the absolute branching fraction to be B(Λ+

c → nKS0π+) = (1.82 ± 0.23(stat) ± 0.11(syst))%. A

comparison to B(Λ+

c → p( ¯Kπ)0) provides an important test of isospin symmetry and final state

122

PACS numbers: 13.30.Eg, 14.20.Lq, 13.66.Bc 124

The ground-state charmed baryon Λ+

c decays

eventu-ally into a proton or a neutron, each taking about half of

126

the total branching fraction (BF) [1]. However, to date no direct measurement of the decay modes involving a

128

neutron has been performed. It has been argued that isospin symmetry works well in the charmed baryon

sec-130

tor [2]. Comparing BFs of the final states with a neutron to the final states with a proton provides an important

132

observable in testing isospin symmetry in Λ+

c three-body

decays [2]. The decay Λ+

c → n ¯K0π+ is the most favored 134

decay of the Λc involving a neutron. Under the isospin

symmetry, its amplitude is related to those of the most

136

favored proton modes Λ+

c → pK−π+ and Λ+c → p ¯K0π0

as A(n ¯K0_π+_{) + A(pK}−_π+_{) +}√_{2A(p ¯K}0_π0_{) = 0. Hence,} 138

precise measurement of the BF for Λ+

c → n ¯K0π+provide

stringent test on the isospin symmetry in the charmed

140

baryon decays by examining this triangle relation. Furthermore, study of Λ+

c → n ¯K0π+ is important to 142

explore the decay mechanism of the Λ+

c, especially the

factorization scheme and the involved final state

interac-144

tion [2, 3]. In the three-body Λ+

c decay to N ¯Kπ, the total

decay amplitudes can be decomposed into two isospin

146

amplitudes of the N ¯K system as isosinglet (I(0)_{) and}

isospin-one (I(1)_{). In the factorization limit, the} color-148

allowed tree diagram, in which the π+_{is emitted and the}

N ¯K is an isosinglet, dominates I(0)_{, and I}(1) _is expect-150

ed to be small compared to I(0) _{as it can only proceed}

through the color-suppressed tree diagrams. Though the

152

factorization scheme is spoiled in charmed meson decays, whether this scheme is valid in the charmed baryon Λ+ c 154

decays is of great interest to both theorists and experi-mentalists and strongly deserves the experimental

inves-156

tigation. The measurement of BF for Λ+

c → n ¯K0π+

can validate or falsify this scheme. Together with the

158

Λ+

c → p( ¯Kπ)0, the Λ+c → n ¯K0π+ can be used to

de-termine the magnitudes of the two isospin amplitudes

160

and their phase difference, which provides crucial infor-mation on the final state interaction. In addition, hight

162

statistics data will facilitate to understand the resonant structures [4, 5] in the three-body Λc decays and test 164

the SU(3) flavor symmetry [2]. Throughout the paper, charge conjugate modes are always implied.

166

This Letter reports on the observation of the final states with a neutron Λ+

c → nKS0π+. The data ana-168

lyzed correspond to 566.93 ± 0.11 pb−1 _{[6] of e}+_e−

an-nihilations accumulated with the BESIII experiment at

170

√

s = 4.599 GeV [7]. This energy is slightly above the mass threshold of a Λ+

cΛ¯−c pair, at which Λ+cΛ¯−c are pro-172

duced in pairs and no additional hadron is kinematical-ly allowed. The anakinematical-lysis technique in this work, which

174

was first applied in the Mark III experiment [8], is spe-cific for charm hadron pairs produced near threshold.

176

First, we select a data sample of ¯Λ−

c baryons by

recon-structing exclusive hadronic decays, called the single tag

178

(ST) sample. Then, we search for Λ+

c → nKS0π+ in

the system recoiling against the ST ¯Λ−

c baryons, called 180

the double tag (DT) sample. In the final state nK0 Sπ+,

the neutron is not detected, and its kinematics is

de-182

duced by four-momenta conservation. The absolute BF of Λ+

c → nKS0π+is then determined from the probability 184

of detecting the process Λ+

c → nKS0π+in the ST sample.

This method provides a clean and straightforward BF

186

measurement independent of the total number of Λ+ cΛ¯−c

events produced.

188

The BESIII detector is a cylindrical detector with a solid-angle coverage of 93% of 4π that operates at the

190

BEPCII collider. It consists of a Helium-gas based main drift chamber (MDC), a plastic scintillator time-of-flight

192

(TOF) system, a CsI (Tl) electromagnetic calorimeter (EMC), a superconducting solenoid providing a 1.0 T

194

magnetic field and a muon counter. The charged particle momentum resolution is 0.5% at a transverse

momen-196

tum of 1 GeV/c. The photon energy resolution in EMC is 2.5% in the barrel and 5.0% in the end-caps at energies

198

of 1 GeV. More details about the design and performance of the detector are given in Ref. [9].

200

A GEANT4-based [10] Monte Carlo (MC) simulation package, which includes a description of the detector

ge-202

ometry and the detector response, is used to determine the detection efficiency and to estimate potential

back-204

grounds. Signal MC samples of a Λ+

c baryon decaying

only to nK0

Sπ+ together with a ¯Λ−c decaying only to the 206

studied tag modes are generated by the MC event gen-erator KKMC [11] using EVTGEN [12], including the

208

effects of initial-state radiation (ISR) [13]. Final-state radiation (FSR) off the charged tracks is simulated with

210

the PHOTOS package [14]. The Λ+c → nKS0π

+ _decay

is simulated using a phase space model since the

two-212

body invariant mass spectra found in data for Mnπ+,

MnK0

S and MK 0

Sπ+ show no obvious structure. To study 214

backgrounds, inclusive MC samples consisting of gener-ic Λ+

cΛ¯−c events, D∗(s)D¯ (∗)

(s) + X production, ISR return 216

to the charmonium(-like) ψ states at lower masses, and QED processes are generated. All decay modes of the

218

Λc, ψ and D(s) as specified in the Particle Data Group

(PDG) [1] are simulated by the EVTGEN MC generator,

220

while the unknown decays of the ψ states are generated with LUNDCHARM [15].

222

The ST ¯Λ−

c baryons are reconstructed using eleven

hadronic decay modes as listed in the first column of

224

Table I, where the intermediate particles K0

S, ¯Λ, ¯Σ0,

¯

Σ− _{and π}0 _{are reconstructed through their decays of} 226 K0 S → π+π−, ¯Λ → ¯pπ+, ¯Σ0 → γ ¯Λ with ¯Λ → ¯pπ+, ¯ Σ−_{→ ¯pπ}0 _{and π}0_{→ γγ, respectively.} 228

Charged tracks are required to have polar angles with-in | cos θ| < 0.93, where θ is the polar angle of the charged

4

track with respect to the beam direction. Their distances of closest approach to the interaction point (IP) are

re-232

quired to be less than 10 cm along the beam direction and less than 1 cm in the perpendicular plane. Tracks

origi-234

nating from K0

S and Λ decays are not subjected to these

distance requirements. To discriminate pions from kaons,

236

the specific ionization energy loss (dE/dx) in the MDC and TOF information are used to obtain particle

identi-238

fication (PID) probabilities for the pion (Lπ) and kaon

(LK) hypotheses. Pion and kaon candidates are selected 240

using Lπ > LK and LK > Lπ, respectively. For proton

identification, information from dE/dx, TOF, and EMC

242

are combined to calculate the PID probability L′_{, and}

a charged track satisfying L′

p > L′π and L′p > L′K is 244

identified as a proton candidate.

Photon candidates are reconstructed from isolated

246

clusters in the EMC in the regions | cos θ| ≤ 0.80 (barrel) and 0.86 ≤ | cos θ| ≤ 0.92 (end cap). The deposited

ener-248

gy of a neutral cluster is required to be larger than 25 (50) MeV in barrel(end cap) region, and the angle between the

250

photon candidate and the nearest charged track must be larger than 10◦_{. To suppress electronic noise and energy} 252

deposits unrelated to the events, the difference between the EMC time and the event start time is required to

254

be within (0, 700) ns. To reconstruct π0 _{candidates, the}

invariant mass of the accepted photon pair is required to

256

be within (0.110, 0.155) GeV/c2_{. A kinematic fit is}

per-formed to constrain the γγ invariant mass to the nominal

258

π0_{mass [1], and the χ}2_{of the kinematic fit is required to}

be less than 20. The fitted momenta of the π0 _{are used} 260

in the further analysis. To reconstruct K0

S and ¯Λ candidates, a vertex-262

constrained fit is applied to π+_π−_{and ¯pπ}+_{combinations,}

and the fitted track parameters are used in the further

264

analysis. The signed decay length L of the secondary vertex to the IP is also required to be larger than

ze-266

ro. The same PID requirements as mentioned before are applied to the proton candidate, but not to the π

can-268

didate. The invariant masses Mπ+_π−, Mpπ¯ +, M_{γ ¯Λ} and

Mpπ¯ 0 are required to be within (0.485, 0.510) GeV/c2, 270

(1.110, 1.121) GeV/c2_{, (1.179, 1.205) GeV/c}2 _and

(1.173, 1.200) GeV/c2 _{to select candidates for K}0 S, ¯Λ, 272

¯

Σ0_{and ¯Σ}− _{candidates, respectively.}

For the ST mode ¯pK0

Sπ0, the backgrounds involving ¯Λ 274

and ¯Σ− _{are rejected by rejecting any event with M} ¯ pπ+∈ (1.105, 1.125) GeV/c2_{and M} ¯ pπ0 ∈ (1.173, 1.200) GeV/c2. 276

For the ST modes of ¯Λπ+_π−_π−_{and ¯Σ}−_π+_π−_{, the}

back-grounds involving K0

S and Λ as intermediate states are 278

suppressed by requiring Mπ+_π− ∈ (0.480, 0.520) GeV/c/ 2

and Mpπ¯ +∈ (1.105, 1.125) GeV/c/ 2. 280

The ST ¯Λ−

c signal candidates are identified using

the variable of beam constrained mass, MBC · c2 ≡ 282 q E2 beam− |−→pΛ¯− c · c| 2_{, where E}

beam is the beam energy

and −→p_Λ¯−_c is the momentum of the ¯Λ −

c candidate. To 284

improve the signal purity, the energy difference ∆E ≡

TABLE I. ST modes, ∆E requirements and ST yields NΛ¯−

c in data. The errors are statistical only.

Mode ∆E (GeV) NΛ¯−_c

¯ pKS0 [−0.025, 0.028] 1066 ± 33 ¯ pK+π− [−0.019, 0.023] 5692 ± 88 ¯ pK0 Sπ0 [−0.035, 0.049] 593 ± 41 ¯ pK+π−π0 [−0.044, 0.052] 1547 ± 61 ¯ pKS0π+π − [−0.029, 0.032] 516 ± 34 ¯ Λπ− [−0.033, 0.035] 593 ± 25 ¯ Λπ− π0 [−0.037, 0.052] 1864 ± 56 ¯ Λπ−π+_π− [−0.028, 0.030] 674 ± 36 ¯ Σ0 π− [−0.029, 0.032] 532 ± 30 ¯ Σ− π0 [−0.038, 0.062] 329 ± 28 ¯ Σ− π+π− [−0.049, 0.054] 1009 ± 57 All tags 14415 ± 159

Ebeam− E_Λ¯−_c for each candidate is required to be with-286

in approximately ±3σ∆E around the ∆E peak, where

σ∆E is the ∆E resolution and E_Λ¯−

c is the reconstructed 288

¯ Λ−

c energy. The explicit ∆E requirements for the

dif-ferent modes are listed in Table I. The yield of each

290

tag mode is obtained from fits to the MBCdistributions

in the signal region (2.280, 2.296) GeV/c2_{, which is the} 292

same as in Ref. [16]. The yields of reconstructed singly tagged ¯Λ−

c baryons are listed in Table I. Finally, we ob-294

tain the total ST yield summed over all 11 modes to be Ntot

¯

Λ−_c = 14415 ± 159, where the error is statistical only. 296

Candidates for the decay Λ+

c → nKS0π+ are

select-ed from the remaining tracks recoiling against the ST

298

¯ Λ−

c candidates. A pion with charge opposite to the ST

¯ Λ−

c is selected, and a KS0 candidate is selected with the 300

same selection criteria as described above but without the Mπ+_π− mass requirement. If more than one K

0 S can-302

didate is formed, the one with the largest decay length significance L/σLis retained, where σL is the vertex res-304

olution of L.

Since the neutron is not detected, we use a kinematic variable

Mmiss2 ≡ Emiss2 /c4− |−→pmiss|2/c2

to obtain information on the missing neutron, where

306

Emiss and ~pmiss are the missing energy and momentum

carried by the neutron, respectively, which are calculated

308 by Emiss≡ Ebeam− EK0 S− Eπ+ and ~pmiss≡ ~pΛ+c − ~pKS0− ~ pπ+, where ~p Λ+

c is the momentum of the Λ + c baryon, EK0 S 310 (~pK0 S) and Eπ

+ (~p_π+) are the energies (momenta) of the

K0

S and π+, respectively. Here, the momentum ~pΛ+ c is 312 given by ~p_Λ+ c = −ˆptag q E2 beam/c2− m2Λ¯− cc 2_{, where ˆp} tagis

the direction of the momentum of the ST ¯Λ−

c and mΛ¯−_c 314

is the nominal ¯Λ−

c mass [1]. If the KS0 and π+ from the

decay Λ+

c → nKS0π+ are correctly identified, the Mmiss2 316

) 4 /c 2 (GeV miss 2 M 0.7 0.8 0.9 1 1.1 ) 2 (GeV/c -π + π M 0.46 0.48 0.5 0.52 0.54

FIG. 1. Scatter plot of Mπ+_π−versus M

2

missfor Λ+c → nKS0π+

observed from data.

squared.

318

The scatter plot of Mπ+_π− versus M 2

missfor the Λ+c →

nK0

Sπ+ candidates in data is shown in Fig. 1, where 320

a cluster of events in the signal region is clearly visi-ble. According to MC simulations, the dominant

back-322

grounds are from the decays Λ+

c → Σ−π+π+ and Λ+c →

Σ+_π+_π− _{with Σ}± _{→ nπ}±_{, which have the same} fi-324

nal state as signal. These background events form a peaking background in M2

miss, but are distributed flat in 326

Mπ+_π−. Backgrounds from non-Λ +

c decays are estimated

by examining the ST candidates in the MBC sideband 328

(2.252, 2.272) GeV/c2 _{in data, whose area is 1.6 times}

larger than the background area in the signal region.

330

To obtain the yield of Λ+

c → nKS0π+ events, we

per-form a two-dimensional unbinned maximum likelihood fit

332

to the M2

miss and Mπ+_π− distributions in both MBC

sig-nal and sideband regions simultaneously. As verified with

334

MC simulations, we model the Mπ+_π− and M 2

miss

distri-butions with a product of two one-dimensional

probabil-336

ity density functions, one for each dimension. The signal functions for M2

miss and Mπ+_π− are both described by 338

double Gaussian functions. The peaking background in the M2

missdistribution is described by a double Gaussian 340

function with parameters fixed according to MC simula-tions, and the flat distribution in the Mπ+_π− spectrum 342

is described by a constant function. The non-Λ+c

de-cay background is modelled by a second-order

polyno-344

mial function in the M2

miss distribution and a Gaussian

function plus a second-order polynomial function in the

346

Mπ+_π− distribution, in which the parameters and the

normalized background yields are constrained by the

348

events in MBC sideband in the simultaneous fit. The

fit procedure is validated by analyzing a large ensemble

350

of MC-simulated samples, in which the pull distribution of the fitted yields is in good agreement with the normal

352

distribution. Projections of the final fit to data are shown in Fig. 2. From the fit, we obtain Nobs

nK0 Sπ

+ = 83.2 ± 10.6, 354

where the error is statistical only.

The absolute branching fraction for Λ+

c → nKS0π+ is 356 4 /c 2 Events/0.010 GeV 10 20 30 4 /c 2 Events/0.010 GeV 10 20 30 4 /c 2 Events/0.010 GeV 10 20 30 _(a) distri M 2 Events/2.5 MeV/c 10 20 30 distri M 2 Events/2.5 MeV/c 10 20 30 distri M 2 Events/2.5 MeV/c 10 20 30 _(b) ) 4 /c 2 (GeV miss 2 M 0.7 0.8 0.9 1 1.1 5 10 ) 4 /c 2 (GeV miss 2 M 0.7 0.8 0.9 1 1.1 5 10 (c) ) 2 (GeV/c -π + π M 0.46 0.48 0.5 0.52 0.54 5 10 ) 2 (GeV/c -π + π M 0.46 0.48 0.5 0.52 0.54 5 10 (d)

FIG. 2. Simultaneous fit to M2

miss and Mπ+_π− of events in

(a, b) the ¯Λ−

c signal region and (c, d) sideband regions. Data

are shown as the dots with error bars. The long-dashed lines

(blue) show the Λ+

c backgrounds while the dot-dashed curves

(pink) show the non-Λ+

c backgrounds. The (red) solid curves

show the total fit. The (yellow) shaded area show the MC

simulated backgrounds from Λ+

c decay. determined by B(Λ+c → nKS0π+) = Nobs nK0 Sπ+ Ntot ¯ Λ−_c × εnKS0π+× B(K 0 S → π+π−) , (1) where εnK0

Sπ+ is the detection efficiency for the Λ +

c →

358

nK0

Sπ+decay, which does not include the branching

frac-tion for K0

S → π+π−. For each ST mode i, the effi-360

ciency ǫi nK0

Sπ+ is obtained by dividing the DT efficiency

ǫi tag,nK0 Sπ+ by the ST efficiency ǫ i tag. Weighting ǫinK0 Sπ+ 362

by the ST yields in data for each tag mode, we obtain ε_nK0

Sπ+= (45.9 ±0.3)%. Inserting the values of N obs nK0 Sπ+, 364 Ntot ¯ Λ−_c, εnK0Sπ+ and B(K 0 S→ π+π−) [1] in Eq. (1), we ob-tain B(Λ+

c → nKS0π+) = (1.82 ± 0.23)%, where the sta-366

tistical error, including those from Nobs nK0 Sπ+ and N tot ¯ Λ−_c is presented. 368

With the DT technique, the systematic uncertainties from the ST side cancel in the branching fraction

mea-370

surement. The systematic uncertainties for measuring B(Λ+

c → nKS0π+) mainly arise from the uncertainties 372

of PID, tracking, K0

S reconstruction and the fit

proce-dure. Throughout this paragraph, all quoted

system-374

atic uncertainties are relative uncertainties. The un-certainties in the π PID and tracking are both

deter-376

mined to be 1.0% by studying a set of control sam-ples of e+_e− _{→ π}+_π−_π+_π−_{, e}+_e− _{→ K}+_K−_π+_π− _and 378

e+_e− _{→ p¯pπ}+_π− _{based on data taken at energies above}

4.0 GeV. The uncertainty in the efficiency of K0 S recon-380

struction is determined to be 1.5% by studying the con-trol samples of J/ψ → K∗∓_K± _{and J/ψ → φK}0

SK±π∓. 382

6

TABLE II. Summary of the relative systematic uncertainties

for B(Λ+ c → nKS0π+). Source Uncertainty π±PID 1.0% π±tracking 1.0% KS0 reconstruction 1.5% Fit 5.2% B(K0 S→ π+π − ) 0.1% Ntot ¯ Λ−_c 1.0% MC statistics 0.6% MC Model 1.3% Total 5.9%

The uncertainty due to the fit procedure is estimat-ed to be 5.2% by varying the fit range, the shapes of

384

background and signal components, and the choice of sideband regions. Besides these uncertainties mentioned

386

above, there are systematic uncertainties from the quot-ed branching fraction for K_S0 → π+π− _{(0.1%), the N}tot ¯ Λ−_c 388

(1.0%) evaluated by using alternative signal shapes in fits to the MBCspectra, the MC statistics (0.6%), the signal 390

MC model (1.3%) estimated by taking into account the statistical variations in the Mnπ+, M

nK0

S and MK 0 Sπ+ 392

spectra observed in data. These systematic uncertainties are summarized in Table II, and the total systematic

er-394

ror is estimated to be 5.9% by adding up all the sources in quadrature.

396

In summary, using 567 pb−1 _{of e}+_e− _{collision data}

taken at √s = 4.599 GeV with the BESIII detector,

398

we report the observation of the decay Λ+

c → nKS0π+.

We measure the absolute branching fraction for Λ+

c →

400

nK0

Sπ+, B(Λ+c → nKS0π+) = (1.82±0.23±0.11)%, where

the first uncertainty is statistical and the second is

sys-402

tematic. This is the first direct measurement of a Λ+ c

decay involving the neutron in the final state since the

404

discovery of the Λ+

c more than 30 years ago. Quoting

B(Λ+

c → pK−π+) and B(Λ+c → pKS0π0) measured by 406

BESIII [17], it can be found that the amplitudes of the above three decay processes satisfy the triangle relation

408

and validate the isospin symmetry [2]. Besides, we obtain B(Λ+

c → n ¯K0π+)/B(Λ+c → pK−π+) = 0.62 ± 0.09 and 410

B(Λ+

c → n ¯K0π+)/B(Λ+c → p ¯K0π0)) = 0.97 ± 0.16 [18],

in which the common uncertainties have been cancelled

412

in the calculation. According to Ref. [2], based on these ratios, the strong phase difference of I(0) _{and I}(1) _is 414

calculated to be cos δ = −0.24 ± 0.08, which is use-ful to understand the final state interactions in Λ+

c

de-416

cays. Furthermore, the relative size of the two ampli-tudes |I(1)_|/|I(0)_{| is evaluated to be 1.14 ± 0.11, which} 418

indicates that the amplitude I(1) _{is not small as}

expect-ed in the factorization scheme. This is consistent with

420

the behaviors in the charmed meson decays [19]. These results will be essential inputs for the study of other Λc 422

decays in theory. Hence, the measurement of the neutron mode in this work provides the first complementary

da-424

ta to the previously measured decays involving a proton, which represents significant progress in studying the Λ+

c. 426

The analysis method used in this work can also be ex-tended to study more decay modes involving a neutron.

428

Lei Li, X.-R. Lyu and H.-L. Ma thank Wei Wang and Fu-Sheng Yu for useful discussions. The BESIII

col-430

laboration thanks the staff of BEPCII and the IHEP computing center for their strong support. This work

432

is supported in part by National Key Basic Research Program of China under Contract No. 2015CB856700;

434

National Natural Science Foundation of China (NSFC) under Contracts Nos. 11235005, 11235011, 11275266,

436

11305090, 11305180, 11322544, 11335008, 11425524, 11505010; the Chinese Academy of Sciences (CAS)

438

Large-Scale Scientific Facility Program; the CAS Center for Excellence in Particle Physics (CCEPP);

440

the Collaborative Innovation Center for Particles and Interactions (CICPI); Joint Large-Scale Scientific Facility

442

Funds of the NSFC and CAS under Contracts Nos. U1232201, U1332201; CAS under Contracts

444

Nos. KJCX2-YW-N29, KJCX2-YW-N45; 100 Talents Program of CAS; National 1000 Talents Program of

446

China; INPAC and Shanghai Key Laboratory for Particle Physics and Cosmology; German Research Foundation

448

DFG under Contracts Nos. Collaborative Research Center CRC 1044, FOR 2359; Istituto Nazionale di Fisica

450

Nucleare, Italy; Joint Large-Scale Scientific Facility Funds of the NSFC and CAS under Contract No.

452

U1532257; Joint Large-Scale Scientific Facility Funds of the NSFC and CAS under Contract No. U1532258;

454

Koninklijke Nederlandse Akademie van Wetenschappen (KNAW) under Contract No. 530-4CDP03; Ministry of

456

Development of Turkey under Contract No. DPT2006K-120470; NSFC under Contract No. 11275266; The

458

Swedish Resarch Council; U. S. Department of Energy under Contracts Nos. DE-FG02-05ER41374,

DE-SC-460

0010504, DE-SC0012069, DESC0010118; U.S. National Science Foundation; University of Groningen (RuG)

462

and the Helmholtzzentrum fuer Schwerionenforschung GmbH (GSI), Darmstadt; WCU Program of National

464

Research Foundation of Korea under Contract No. R32-2008-000-10155-0. This paper is also supported by

466

the Beijing municipal government under Contract Nos. KM201610017009, 2015000020124G064.

468

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